Method and Apparatus for Mixed Analog/Digital Beamforming

1. A method for mixed analog/digital beamforming by a transmitter in a mobile communication system, the method comprising: converting a modulation symbol coded and modulated according to a predetermined modulation level into a parallel symbol stream; performing digital beamforming on the parallel symbol stream by multiplying a second subcarrier to be transmitted to a second receiver by a digital beamforming precoder, the digital beamforming precoder determined to compensate the second subcarrier for an analog beamforming precoder, the analog beamforming precoder optimized for a first subcarrier to be transmitted to a first receiver; performing an inverse fast Fourier transform (IFFT) operation on the digitally-beamformed parallel symbol stream to generate a time-domain symbol; converting the IFFT-operated time-domain symbol into a serial time-domain symbol; inserting a cyclic prefix (CP) into the serial time-domain symbol; performing a digital-to-analog converter (DAC) operation on the CP-inserted symbol to generate an analog signal; and performing analog beamforming by multiplying the analog signal by the analog beamforming precoder optimized for the first subcarrier, the first subcarrier transmitted through at least one radio frequency (RF) channel.

2. The method of claim 1 , further comprising determining the digital beamforming precoder and the analog beamforming precoder.

3. The method of claim 2 , wherein determining the digital beamforming precoder and the analog beamforming precoder comprises determining an optimized digital beamforming precoder and an optimized analog beamforming precoder optimized for a receiver based on a sounding signal transmitted by the receiver.

4. The method of claim 2 , wherein determining the digital beamforming precoder and the analog beamforming precoder comprises determining the digital beamforming precoder and the analog beamforming precoder according to feedback from a receiver measuring a reference signal transmitted by the transmitter.

5. The method of claim 4 , wherein the feedback includes at least one of a codebook index, a beamforming matrix, and a beamforming precoder.

6. The method of claim 1 , wherein the at least one RF channel is connected to an antenna subset of all antennas.

7. The method of claim 1 , wherein the digital beamforming precoder is indicated by a digital phase offset determined to compensate the second subcarrier to be transmitted to the second receiver.

8. The method of claim 1 , wherein the analog beamforming precoder is indicated by an analog phase offset optimized for the first subcarrier transmitted through at least one RF channel.

9. An apparatus of a transmitter configured to perform mixed analog/digital beamforming in a mobile communication system, the apparatus comprising: a serial-to-parallel converter (SPC) unit configured to convert a modulation symbol coded and modulated according to a predetermined modulation level into a parallel symbol stream; a precoder configured to perform digital beamforming on the parallel symbol stream by multiplying a second subcarrier to be transmitted to a second receiver by a digital beamforming precoder, the digital beamforming precoder determined to compensate the second subcarrier for an analog beamforming precoder, the analog beamforming precoder optimized for a first subcarrier to be transmitted to a first receiver; an inverse fast Fourier transform (IFFT) unit configured to perform an IFFT operation on the digitally-beamformed parallel symbol stream to generate a time-domain symbol; a parallel-to-serial converter (PSC) unit configured to convert the IFFT-operated time-domain symbol into a serial time-domain symbol; a cyclic prefix (CP) inserter configured to insert a CP into the serial time-domain symbol; a digital-to-analog converter (DAC) unit configured to perform a DAC operation on the CP-inserted symbol to generate an analog signal; and a radio frequency (RF) unit configured to perform analog beamforming by multiplying the analog signal by the analog beamforming precoder optimized for the first subcarrier, the first subcarrier transmitted through at least one RF channel.

10. The apparatus of claim 9 , further comprising a control unit configured to determine the digital beamforming precoder and the analog beamforming precoder.

11. The apparatus of claim 10 , wherein to determine the digital beamforming precoder and the analog beamforming precoder, the control unit is configured to determine the digital beamforming precoder and the analog beamforming precoder optimized for a receiver based on a sounding signal transmitted by the receiver.

12. The apparatus of claim 10 , wherein to determine the digital beamforming precoder and the analog beamforming precoder, the control unit is configured to determine the digital beamforming precoder and the analog beamforming precoder according to a feedback from a receiver measuring a reference signal transmitted by the transmitter.

13. The apparatus of claim 12 , wherein the feedback comprises at least one of a codebook index, a beamforming matrix, and a beamforming precoder.

14. The apparatus of claim 10 , wherein the transmitter comprises a plurality of RF chains connected to respective antennas, and wherein each of the RF chains comprises a phase shifter configured to perform analog beamforming on a signal transmitted through each antenna according to a control signal of the control unit.

15. The apparatus of claim 10 , wherein the transmitter comprises a switch configured to connect a transmitting/receiving (TX/RX) modem and each of a plurality of antennas, and wherein the switch is configured to connect the TX/RX modem and at least one antenna according to a control signal of the control unit.

16. The apparatus of claim 15 , wherein the control unit is configured to switch on at least one of the plurality of antennas.

17. The apparatus of claim 9 , wherein the at least one RF channel is connected to an antenna subset of all the plurality of antennas.

18. The apparatus of claim 9 , wherein the digital beamforming precoder is indicated by a digital phase offset determined to compensate the second subcarrier to be transmitted to the second receiver.

19. The apparatus of claim 9 , wherein the analog beamforming precoder is indicated by an analog phase offset optimized for the first subcarrier transmitted through at least one RF channel.

20. A method for operating a receiver in a mobile communication system, the method comprising: measuring a reference signal transmitted by a transmitter; transmitting feedback information generated based on the measurement; and receiving a data signal which is beamformed by a digital beamforming precoder and an analog beamforming precoder on a first subcarrier, wherein the data signal is beamformed, at the transmitter, by multiplying the data signal to be transmitted to the receiver by the digital beamforming precoder, and wherein the digital beamforming precoder is determined to compensate the first subcarrier for another analog beamforming precoder for a second subcarrier to be transmitted to another receiver.

21. A method for operating a receiver in a mobile communication system, the method comprising: transmitting a sounding signal to a transmitter; and receiving a data signal which is beamformed by a digital beamforming precoder and an analog beamforming precoder on a first subcarrier, wherein the digital beamforming precoder and the analog beamforming precoder is determined based on the sounding signal at the transmitter, wherein the data signal is beamformed, at the transmitter, by multiplying a signal to be transmitted to the receiver by the digital beamforming precoder, and wherein the digital beamforming precoder is determined to compensate the first subcarrier for another analog beamforming precoder for a second subcarrier to be transmitted to another receiver.

22. The method of claim 20 , wherein the feedback information comprises at least one of a codebook index, a beamforming matrix, and a beamforming precoder.

23. An apparatus of a receiver in a mobile communication system, the comprising: a controller for measuring a reference signal transmitted by a transmitter; a transmitting unit for transmitting feedback information generated based on the measurement; and a receiving unit for receiving a data signal which is beamformed by a digital beamforming precoder and an analog beamforming precoder on a first subcarrier, wherein the data signal is beamformed, at the transmitter, by multiplying the data signal to be transmitted to the receiver by the digital beamforming precoder, and wherein the digital beamforming precoder is determined to compensate the first subcarrier for another analog beamforming precoder for a second subcarrier to be transmitted to another receiver.

24. An apparatus of a receiver in a mobile communication system, the apparatus comprising: a transmitting unit configured to transmit a sounding signal to a transmitter; and a receiving unit configured to receive a data signal which is beamformed by a digital beamforming precoder and an analog beamforming precoder on a first subcarrier, wherein the digital beamforming precoder and the analog beamforming precoder is determined based on the sounding signal at the transmitter, wherein the data signal is beamformed, at the transmitter, by multiplying a signal to be transmitted to the receiver by the digital beamforming precoder, and wherein the digital beamforming precoder determined to compensate the first subcarrier for another analog beamforming precoder for a second subcarrier to be transmitted to another receiver.

25. The apparatus of claim 23 , wherein the feedback information comprises at least one of a codebook index, a beamforming matrix, and a beamforming precoder.